The field of the invention is combined coating processes and the invention is particularly concerned with preparing a multilayer coating consisting of at least one base coating containing pigments and at least one transparent top coating, the multilayer coating being applied to a substrate.
It is known to provide substrates with multilayer coatings as disclosed in U.S. Pat. Nos. 3,639,147 and 4,220,679, the disclosures of which are incorporated herein. Metal and plastic substrates are suitable for these multilayer processes. In particular, the present invention relates to a process for the deposition of a multilayer coating on automobile bodies or parts thereof which are made of metal or plastic and which as a rule are pretreated with a primer system. Until now, automobile bodies have been predominantly made of metal, however, bodies or parts thereof made of plastic are explicitly included herein as substrates.
Coatings deposited on substrates are both for protective and decorative purposes. The coating compositions used in the preparation of such coatings frequently contain pigments which improve the protective and decorative effect of the coatings. The term pigment herein means colored and hueless, organic and inorganic pigments, filler or dyestuffs which are soluble or insoluble in solvents or vehicles. Especially as regards automobile enameling, metallic pigments have for some time been widely used. They offer a varying reflection of incident light as a function of the angle of observation. This effect is frequently called the "flip-flop" effect and depends on the orientation of the flake-like metallic pigments in the finished coating.
Great efforts are exerted when making metallic-effect coatings that an optimal orientation of the metallic pigments be achieved. Until now, coating compositions based on acrylate polymers or polyesters containing hydroxyl groups have been used for such purposes in combination with the conventional crosslinking agents and in organic solvents containing cellulose esters, for instance cellulose acetatebutyrate and other enameling accessories. These known coating compositions suffer from the drawback that their solid content is low and hence their organic solvent content is high. This is uneconomical and results in appreciable stress on the ecology. Another drawback is that the metallic pigments settle markedly and are difficult to stir again into the orginial state.
It has been proposed to solve the above cited problems by the use of microparticles in the coating composition. The term microparticles means polymer particles insoluble in a coating composition. U.S. Pat. No. 4,220,679 discloses a process for the preparation of a protective and/or decorative multilayer coating on a substrate, wherein the base coat deposited is a coating composition containing polymeric microparticles having a diameter of 0.01 to 10 microns. The microparticles are stably dispersed by a steric stabilization (steric barrier). The steric stabilization is implemented by polymer chains arranged around the particles and these polymer chains are solvated by the solvent of a film-forming polymer system. The microparticles therefore are provided with a shell or a boundary zone of polymer material anchored by primary and/or secondary valences to the particle core. Again, these coating compositions which are known from U.S. Pat. No. 4,220,679, tend to have strong settling of the metallic pigments, thereby degrading the metallic effect of the finished coatings.